Thieves will begin to expect it to be there some of the time, after sufficient police/vigilante response. And perhaps it will be hard to find. Hence, deterrence.

Cars are stolen far less often than they would be otherwise partially due to technology like LoJack. Did you grow up in the US? Steering wheel locks used to be on every other parked car. Immobilizers and LoJack made them a thing you buy for your weekend classic car only.